The simplest type of drainage by underground tiling is constructed by excavating a trench, smoothing the bottom to the desired gradient, butt joining the drain tiles together and then back filling the trench. Water from the affected area drains into the tiles through the joints, and, to a lesser extent, through any openings in the walls, and then flows inside the tiles to the drain outlet. Under favorable conditions, such an elementary drain system can function for a very long period of time. However, dirt falling into the tiles through the joints or entering the drain system with the water may fill or plug low spots left by the subsidence of the bottom of the trench.
A better drain system employs a base of an aggregate, such as gravel, upon which the tiles are laid. This provides a firmer foundation or footing for the tiles. In addition, the footing provides a porous space into which dirt or silt can empty through the joints from the pipe. This storage space for silt may fill so that the pipe itself will ultimately fill with silt. However, the footing of gravel serves to trap the dirt brought into the drainage system during the initial period of adjustment after the system is installed. After this adjustment or settling period, it has been observed that little or no dirt flows into the drainage system.
A relatively large amount of land and drain tile is used in farm land applications. The work is usually done on a fairly large scale, on regular grades, and with adequate space. Costs must generally be kept to a minimum if the installation is to be economically justified at all. Ditching machines, backhoes, and excavators are particularly adapted to this kind of work. However, the rate of digging depends on a number of variables, including: depth and width of the trench, bucket size and efficiency, cycle time of the backhoe or excavator, the quality of the soil to be removed, obstacles and hazards both above and below ground, the presence of rock, the accuracy of the grade required, and the need for separating top soil. For a given site and drain system, the bucket size and efficiency are the only variables that can be readily controlled by the backhoe operator.
Small machines, with buckets as narrow as six or eight inches, may be used for drain pipe installations having a depth up to four feet. Buckets of that size produce the minimum excavation needed and insure adequate room for lining up the tile. As the maximum depth is approached greater difficulties are experienced. It becomes more and more difficult to locate drain tile accurately. It is also very difficult to remove earth and rock that falls from the sides of the trench into the bottom of the trench. Accurate gravel placement within the footing or around the tile also becomes inconvenient. A wider bucket per se will eliminate some of these difficulties but more dirt will have to be excavated from the beginning and more backfill applied after the tiles are laid. This raises the cost of the installation. Thus, a mere change in bucket size is not enough.
Typically a supply of tile is laid on the field, parallel with the trench line, just far enough to clear the excavator or backhoe, on the side away from the intended spoil pile. The pieces of tile are laid end-to-end to give the correct number, with a few extra placed at frequent intervals to make up for broken and imperfect tiles. Next the footing is dug into the bottom or base of the trench. If the pipe or tiles are small the footing may be dug manually. However, if the tiles are small and the trench is deep, that may be impossible to do. Consequently, a machine is almost always used to dig the footing. Conventionally this is done by changing the size of the bucket or using a second backhoe with a smaller bucket. Regardless of the option selected costs are increased, either due to the slower rate of laying tile if the bucket is periodically changed, or due to the need to employ a second machine and a second machine operator.
After the footing is dug, the footing is filled with gravel or crushed stone. This material is usually supplied by a dump truck with a small opening in the rear gate. The truck straddles the ditch as the gravel is applied. Next the layer of stone or gravel is smoothed off. Finally, the tile is placed on the trench bottom as soon as possible to minimize the danger of "losing" the trench through caving-in of the sides.
Thus, it should be apparent that the basic trench digging and tile laying procedure just described has room for improvement. If something could be provided that would simplify the procedure or improve the utilization of the machinery used to dig the trenches and the footing for drain tiles, the overall productivity of the backhoe or excavator used to dig the trench would be improved. More importantly, the overall cost of the drainage system would be reduced.